Calretinin-immunoreactive cells and fibers in the human amygdaloid complex.
Calretinin is a calcium-binding protein that colocalizes with GABA in the cerebral cortex and hippocampus of the rat and the monkey. In the present study, we investigated the distribution of calretinin-immunoreactive cells and fibers in the human amygdaloid complex. A conspicuous feature was the high density of calretinin neurons in the human amygdala. The highest densities of the calretinin-immunoreactive neurons were observed in the anterior cortical nucleus, accessory basal nucleus, amygdalohippocampal area, and in the nucleus of the lateral olfactory tract. The paralaminar nucleus, central nucleus, medial nucleus, and the periamygdaloid cortex contained the lowest densities of calretinin neurons. In most of the amygdaloid areas, the calretinin cells had the appearance of aspiny or sparsely spiny local circuit neurons. However, in the amygdalohippocampal area, we found also densely spined dendrites. The cortical areas and the central nucleus were characterized by intense neuropil labeling, while the deep nuclei contained a high density of calretinin-immunoreactive fibers and terminals. Calretinin immunoreactivity was also found in the intra-amygdaloid fiber bundles, stria terminalis, and in the ventral amygdalofugal pathway. This suggests that in addition to the local circuit neurons, calretinin immunoreactivity is also located in neurons that connect the amygdaloid complex with the other brain areas. The distribution and morphological characteristics of calretinin-immunoreactive neurons differed from those of another calcium-binding protein, parvalbumin, in the human amygdala (Sorvari et al.  J. Comp. Neurol. 360:185-212). This suggests that these two calcium-binding proteins are located in different populations of neurons.